1. Field of the Invention
The present invention relates to image processing which can prevent illegal copy of a document whose reproduction is prohibited such as a confidential document or leakage thereof to the outside.
2. Description of the Background Art
Recently, electrophotographic copying machines have been spread widely, and characters or an image printed on a sheet of paper or the like can easily be copied by any person by using such a copying machine. Image processing technology and image forming technology are also improved significantly. Then, especially, by using a state of the art digital color copying machine (MFP), a copy difficult to be distinguished from an original can be created easily.
As explained above, when an important document or a confidential document which has to be prohibited from being duplicated is copied, a copy which is almost the same as the original (document) can be produced. Therefore, it is a problem that an original copied illegally is leaked to the outside of a company or the like. Then, it is an important element for a copying machine or the like to provide measures for preventing forgery, falsification and illegal copying of a printed material such as an important document. In some methods proposed as the measures therefor (for example, Japanese Patent laid open Publications Nos. 6-125459/1994, 2001-86330, and 7-36317/1995), characteristics which would be included in an original to indicate that it is a duplication prohibited document have been registered. When a copy of an original is produced, a characteristic extracted from an input image is compared with each of the registered characteristics, and when one of the characteristics is found, it is determined that the original should not be reproduced. Thus, a copy of an original including such a characteristic is prevented.
However, in the conventional technique using registered characteristics, it is necessary to previously register all of the characteristics of originals or documents which should be prohibited from being duplicated. Therefore, it can only be applied to certain limited documents. Further, because a device for identifying the characteristics by comparison has to be incorporated, a complex technique has to be incorporated. In addition, if a copying machine does not incorporate the technique, it outputs a copy which cannot be distinguished from the original.
It is also proposed to include a latent image in a document prohibited from being duplicated (for example, Japanese Patent laid open Publications Nos. 2001-197297 and 2004-201068). When the document is copied, a latent image which cannot be recognized with naked eyes in the document becomes visible in the copy so as not to output the same copy as the original (document). Thus, illegal copying can be psychologically prevented.
Though this technique can be applied to a general copying ma chine, a copy itself is output. Therefore, though the latent image is visualized, the psychological effect to restrain illegal copying is small if only the visualization of the latent image is used.